1. Field of the Invention
The present invention relates to a control apparatus for a dual clutch transmission, i.e., a transmission including a plurality of clutches, and to a control method for the dual clutch transmission.
2. Description of the Related Art
A conventional dual clutch transmission, which is mounted on a motor vehicle and includes a plurality of clutches for fast gear changes of the motor vehicle, (referred to simply as “transmission” below in some cases) and a control apparatus to control the dual clutch transmission are known (see, for example, Japanese Patent Application Laid-Open No. 2004-251456 and Japanese Patent Application Laid-Open No. 2004-308841).
This transmission includes a plurality of clutches disposed between an engine and input shafts of the two transmission systems, each clutch being capable of transmitting a driving force or blocking the transmission of the driving force, and pairs of gears divided into the two systems (e.g., two systems of an odd transmission stage group and an even transmission stage group) that selectively connect between the input shafts of the two systems and a transmission output shaft (hereinafter referred to simply as “output shaft”) according to a shifter selection.
In this transmission, while power is being transmitted to one of the pairs of gears in one transmission stage group that is connected to one of the clutches by engaging the one clutch and is selected by the shifter, the pairs of gears in the other transmission stage group corresponding to the other clutch can be kept in a neutral state, where no power is transmitted, by selecting with the shifter while the other clutch is held in engagement. That is, in a state where the two clutches are engaged, power can be transmitted from the output shaft to a drive wheel through the desired pair of gears in the transmission stage selected by the shifter.
Also, at the time of changing gears, the other clutch in the plurality of clutches associated with the transmission system of the transmission stage group not transmitting power is released. One of the pairs of gears in the transmission system including the input shaft connected to the other clutch is then selected and shifted into the target transmission stage. The other clutch is thereafter engaged while the one clutch that has been transmitting power is released.
That is, in the conventional dual clutch transmission, when a transmission stage is shifted from a selected and operational transmission stage (also referred to as “previous stage”) to a target transmission stage (also referred to as “next stage”) , a control apparatus performs interchange of the clutches by simultaneously operating the clutches, i.e., by releasing the clutch that is transmitting power and by engaging the clutch that transmits power to be transmitted through the following transmission stage (next stage) in a state where the pairs of gears in the two transmission systems are connected.
As described in Japanese Patent Application Laid-Open No. 2004-251456 and Japanese Patent Application Laid-Open No. 2004-308841, when controlling the conventional dual clutch transmission in which interchange between the plurality of clutches is performed while the clutches are being operated simultaneously, it is necessary to keep the sum of the torque capacities of the clutches simultaneously operated equal to a target value such as a value of the engine torque on a clutch portion from beginning to end of clutch interchange in order to avoid shift shocks (including a change in vehicle speed, and pitching) caused by the interchange as effectively as possible.
In the following description, a clutch that is being used and that is to be interchanged is referred to as a “release-side clutch”, and a clutch that transmits a torque in engagement after gear change is referred to as an “engaging-side clutch”.
Japanese Patent Application Laid-Open No. 2000-55086 discloses a clutch apparatus including a back torque limiter (referred to as “BTL” below as occasion demands) that relieves a back torque transmitted backward from the transmission apparatus to inhibit transmission of an excessively large back torque to the crank shaft.
There is a possibility of occurrence of immobility in the clutches such that the clutch does not move from the current state. Immobility is sometimes called “clutch sticking” (referred to as “sticking” below). The term sticking refers to immobility from the current state rather than adherence of a clutch. For example, the state of the clutch does not change from a state of being held out of engagement, the clutch remains out of engagement, and from a state of being held in engagement, the clutch remains engaged. There is a possibility of occurrence of the sticking in each of the release-side clutch and the engaging-side clutch. If sticking occurs in the release-side clutch, double engagement is caused by the BTL, so that braking is effected and a particularly large clutch malfunction does not occur.
In some cases of a dual clutch transmission including clutches with a BTL, however, when the engaging-side clutch is malfunctioning (sticking mechanically), and when the release-side gears are out of engagement, there is a possibility of failure to transmit a sufficiently large torque even if a control is performed to engage the two clutches.
In a case where the engaging-side clutch is sticking and an attempt is made to perform a final withdrawal operation after disengaging the gears (for example, after making a downshift from the second gear to the first gear), reengagement on the release side by disengaging the gears in the previous stage (second gear side in this case) cannot be performed after the transmission enters an inertia phase. That is, when the final withdrawal operation is performed after the gears are disengaged, the connection in the clutch can be established but the connection through the gears cannot be established since the gears are out of engagement. That is, since dogs are out of engagement and since the engaging-side clutch is sticking, a torque cannot be transmitted even if an attempt is made to establish the connection. This does not mean that a problem occurs immediately, since the transmission is in a power-off downshifting state. However, a subsequent attempt to accelerate brings about a state of the reduced torque on the engaging side, which gives a feeling as if the driving force is lost.